Announcements 1/12/10

Prayer Extra-credit lecture, “The Physics of Bicycles”, right

after class today, 2 pm, 455 MARB. If you attend, turn in 1-page (max) summary to get extra credit points.

I won’t have office hours today. For HW problem 4-9 (extra credit), you need the bulk

modulus of copper. You should get that from Chapter 12 of your book (if you don’t have that chapter, there are some books available in the Tutorial Lab).

I’m not following books order for Chapters 20-22. Be sure to look at syllabus.

Don’t forget to turn in work for ALL homework problems

(1+x)n ≈ 1 + nx

Reading quiz (graded):

Which two temperature scales have the same sized intervals, varying only in their choice of the zero point?

a. Fahrenheit and Celsiusb. Fahrenheit and Kelvinc. Kelvin and Celsiusd. Kelvin and BTUe. Fahrenheit and Pentatonic

Temperature

Temperature scales

What’s a thermometer?

Demos:

Liquid bulb thermometer Constant volume thermometer

Thermal contact Two objects in “thermal contact” will come

to “thermal equilibrium”, and then have the same “temperature”.

What is thermal contact?

What is thermal equilibrium?

What is temperature?a. Is there a maximum temperature?b. Is there a minimum temperature?

Thermal expansion

What went wrong here?

Thermal Expansion

The equation:

= “________________”

0L L T

For reference: steel = 11 10-6 /C

Videos/Demo:

Demo: Ring & Ball Demo: Bimetallic strip Video: Bimetallic strip

Thought question (ungraded)

You heat a disc with a hole in it. Will the radius of the hole get larger, smaller, or stay the same?

a. Largerb. Smallerc. Stay the same

Thought question (ungraded):

If the expansion of all of the linear dimensions of an object is proportional to ΔT, what should the expansion of the surface area of the object be proportional to?

a. ΔTb. 2ΔTc. ΔT2

d. The surface area won’t change with temperature.

e. None of the above

Area & Volume Expansion (of solids)

The equations:

0

0

A A T

V V T

= … = …

Thought question (ungraded):

Two jars of gas: helium and neon. Both have the same volume, same pressure, same temperature. Which jar contains the greatest number of gas molecules? (The mass of a neon molecule is greater than the mass of a helium molecule.)

a. jar of heliumb. jar of neonc. same number

Ideal Gas Law

Hold T constant, then V decreases as P increases

Hold P constant, then V increases as T increases

Hold P, T, constant, then V increases as #molecules increases

Summary:

Ralph is confused…the book calls two different equations “the ideal gas law”: “PV = nRT”, and “PV = NkBT”. Why are they both called the ideal gas law, when only the first equation looks like what he learned in chemistry?

Quick Writing

Important stuff: P must be in _______V must be in _______T must be in _______n = ________R = ________N = ________kB = ________

What’s a mole?How are R and kB related?

Molecules collide like superballs (elastic) due to repulsive forces

No attractive forces Never condense into liquids or solids Are like “frictionless surfaces”, “massless

pulleys”, fluids without viscosity, projectiles without air resistance, etc.

That is, they don’t really exist, but are useful constructs

Ideal Gases:

Demos/Videos:

Video: “barrel crush” Demos: More liquid nitrogen!

a. Two balloonsb. Rubber nailc. “Balloon pop”d. Expansion ratio